Ductless mini-splits: Are they right for you?

In homes with no existing ductwork, ductless mini-splits are often the easiest way to add a permanent cooling system. They’re also a practical solution for sheds, bonus rooms, or any other small space that lacks an HVAC system.

Compared to noisy window AC units and the inefficiency and safety concerns of space heaters, mini-splits are a clear upgrade. While there are some good reasons to consider adding ductwork or connecting an air-to-water heat pump to existing radiators, mini-splits are tough to beat in terms of cost, convenience, and comfort.

You can opt for a mini-split that serves a single room, or install an elaborate system that covers every room in the house. 

Every mini-split system consists of an outdoor unit, often called a condenser or compressor, and one or more indoor units, usually called heads. Each head is an independent heating or cooling zone, allowing you to set different temperatures in different rooms or simply turn off certain heads while leaving the others running.

While the heads usually mount high on a wall, they also come in variants that mount inside a ceiling, or low on a wall, or even in a very short run of ductwork. "Each has its pros and cons around aesthetics, installation dynamics, and price," Josh Lake, co-founder of Elephant Energy. Said to EnergySage. "Ceiling cassettes are the most requested."

Like any heat pump, mini-splits use a trick of physics to cut the total energy use by about two-thirds on average, compared to a traditional fossil fuel furnace or a "regular" electric resistance heater. (In cooling mode, it's no different from a typical air conditioner.)

This doesn't mean you'll always save money compared to gas heating, but you might—and you'll almost certainly cut your carbon footprint.

Mini-splits (and other high-performance heat pumps) can adjust their power output, which often makes them much more comfortable than traditional HVAC systems. This comfort advantage holds year-round, even in very cold weather.

The variable-speed design in modern mini-splits also keeps you comfier in mild weather. In both heating and cooling modes, they're built to maintain a steady, consistent temperature (and humidity), rather than switching on and off constantly between full blasts, as traditional HVAC systems do.

Ductless mini-split heads do have filters, but they're just mesh screens meant to protect the system from getting jammed—not media filters that meaningfully clean the air. Ducted HVAC has a considerable advantage here, in that it's usually incredibly easy to equip those systems with an excellent filter.

This isn't because mini-split manufacturers are being cheap: It's actually a major physics challenge to put a decent filter into a system that's as quiet and compact as a ductless head.

Poor heat pump maintenance can exacerbate the problem. Dust that passes through the mesh filter can combine with moisture condensing on coils or fan blades, creating a dark, damp environment that’s ideal for mold growth. While deep cleaning is possible, it’s tedious due to the compact, all-plastic design.

Some manufacturers have launched or plan to launch new designs to prevent mold growth, but in the meantime, running an additional air cleaner can help. Even a DIY solution, like attaching a high-quality filter to a box fan, can noticeably improve air quality indoors.

Traditional heating equipment runs hot—think 120 degrees Fahrenheit or higher. That intense heat not only feels toasty but can also mask insulation shortcomings, especially with radiators that directly warm walls as well as the air.

By contrast, heat pumps produce milder heat. Cold-climate models typically reach a maximum of around 120 degrees, while basic single-stage systems often operate at 90 to 100 degrees. This is sufficient to keep a home warm, but it can take longer to “catch up” in the mornings if you lower your thermostat overnight. Some experts even recommend minimizing or skipping setbacks entirely with heat pumps.

The milder heat from a mini-split can feel different, particularly near poorly insulated walls or cold corners, where occasional blasts from radiant heat might otherwise mask the drafty feel. For most homes, this is the exception rather than the rule. Upgrading your insulation is generally a good idea, anyway (and some rebate programs actually require this step before they'll give out free money for installing a heat pump).

Mini-splits—and high-efficiency heat pumps in general—tend to be just okay at dehumidification. It's a side effect of their super-efficient energy use: A mini-split's indoor coil doesn't get as cold as a basic AC's coil, so moisture doesn't condense onto it as readily.

Realistically, this probably only matters a couple of days per year. It’s a non-issue in the summer, given the high temperatures keep mini-splits running constantly, and when they’re always cooling, they're also always dehumidifying. The tricky part comes in the shoulder seasons, when it’s mild but the dew points are high. In those cases, the mini-split might not run long enough to keep moisture in check.

The good news is that whole-home mini-splits aren’t common in regions where this poses a legitimate problem, like the Southeast. They’re more often installed in places like New England, where high dew points in spring and fall are rare.

There are a couple of easy fixes: Pick a mini-split with a “Dry” setting, which dehumidifies while slightly reheating to maintain a steady temperature, or just run a standalone dehumidifier on the days it matters.

Despite the promise of room-by-room control, multi-zone mini-splits tend to work better if all the zones are running simultaneously—but not everyone wants to use their mini-splits that way, and in that case, the designer can help figure out a workaround.

The outdoor unit’s capacity should roughly match the whole house, and each indoor unit should be sized for the room it serves. That said, there are a few common challenges designers often run into.

For example, a small bedroom might only need 2,800 Btu of cooling, but the smallest indoor heads start at 5,000 Btu. On the other end of the spectrum, very large rooms can make it difficult for conditioned air to circulate evenly. If airflow gets trapped in one corner, the unit can short-cycle, which reduces comfort and efficiency.

Another consideration is placement. Not every room has a good spot for a standard high-wall head, so designers may need to choose a different style, like a ceiling cassette or a floor unit. 

Whatever the case, there's almost always a solution, but the designer might have to spend some time thinking it through.

The secret to the best performance here is having a low minimum capacity, relative to the maximum output—also known as a turndown ratio. 

"A big turndown is incredibly important for mini-splits," said Peter Freedman, an HVAC designer from Massachusetts with about a decade of mini-split experience. The outdoor unit determines the system’s power output, so each individual indoor head can only be turned down so low—which might not be the ideal level for comfort and energy efficiency. 

The wider the gap between the lowest and highest capacities, the more leeway the system has to operate effectively across a greater number of indoor units. Some popular systems turn down to about one-third of their maximum capacity, while others have a wider range down to about one-fourth of the max—enough to be a difference-maker.

Depending on the brand, you can connect anywhere from five to eight indoor units to a single outdoor unit (ODU)—often enough to cover every part of a typical single-family house. Still, many whole-home setups utilize two outdoor units, each with two or three indoor heads attached. 

There are pros and cons to both approaches: Single ODU setups take up less space on your property and your electrical panel, and can cost a little less. However, running all the refrigerant lines from a single outdoor location can present some challenges. Multi-ODU setups can be more energy efficient and can avoid the challenges of those long refrigerant runs. But, they take up more space, and we've seen several cases where they're badly oversized.